1. Field of the Invention
This invention relates generally to packaged semiconductor devices. More particularly, the invention pertains to flash memory devices having high memory density.
2. State of the Art
The use of semiconductor integrated circuit (IC) chips is widespread, in both commercial grade and specific high reliability applications. Continuing progress in the manufacture of IC chips has resulted in chips of greatly increased density, i.e., higher number of devices per footprint area of each chip. In addition, to produce increasingly complex electronic components, it is necessary to include a much greater number of IC chips on a substrate, e.g., a circuit board. One solution to this dilemma is to form a stack of chips on a substrate, creating what is known in the art as a multi-chip package.
The state of the art in vertically stacked multi-chip module (MCM) devices is illustrated by representative prior art devices shown in drawing FIGS. 1 through 11.
A representative example of a known multi-chip module semiconductor device 10, prior to packaging, is shown in drawing FIG. 1.
A plurality of chips or dice 12A, 12B and 12C are mounted in a pyramidal stack on a substrate 14. Each die is mounted with an adhesive material 16 to the next lower die or substrate and is electrically connected to the metalized substrate 14 by bond wires 18 using known wire bonding methods. Variants of this multi-chip configuration are described in U.S. Pat. No. 5,422,435 to Takiar et al., Japan Patent 62-8534(A) to Tsukahara and Japan Patent 3-165550(A) to Yashiro. In each of these references, a pyramidal stack is formed of increasingly smaller chips or dice 12, in order to accommodate the placement of bond wires 18 on peripheral portions of each die 12. This configuration is not generally useful where dice of equal dimensions are to be placed in a multi-chip module (MCM), such as in a memory device.
In drawing FIG. 2, a pyramidal stack of chips in device 10 is shown as described in U.S. Pat. No. 5,399,898 to Rostoker. In these references, the dice 12A, 12B, 12C comprise “flip-chips” with solder bumps or balls 20 joined to conductive areas on the back side 22 of the underlying chip.
Depicted in drawing FIG. 3 is an MCM device 10 in which a first die 12A is attached to a substrate 14 with adhesive 16 and is electrically connected to the substrate 14 with bond wires 18. A second die 12B is stacked atop the first die 12A and connected to it by solder balls 20. The second die 12B is smaller than the first die 12A, in order to leave access to the first die's 12A conductive areas. This type of arrangement is depicted in Japan Patent 56-158467(A) to Tsubouchi, and a variant thereof is described in Japan Patent 63-104343 to Kuranaga.
Depicted in drawing FIG. 4 is an MCM device 10 formed of dice 12A and 12D mounted on opposite surfaces of a substrate 14. In this example, the substrate 14 is a lead frame, and the construction permits both of the dice 12A, 12D to be connected to a metallization on one surface of the lead frame. This construction is described in U.S. Pat. No. 5,012,323 to Farnworth.
Each of the above stacking configurations requires that the dice be of differing sizes. This is mandated by the need to leave the bond pads of each die unobstructed for wire attachment.
There have been various configurations of MCM devices in which chips of equal dimensions are stacked. Several such configurations are shown in drawing FIGS. 5, 6, 7, 8, 9, 10 and 11 and described below.
In one MCM device configuration shown in U.S. Pat. No. 5,973,403 to Wark and Japan Patent 5-13665(A) to Yamauchi, a flip-chip 12A is electrically bonded to a substrate 14 by posts, balls or other connectors 20, and a second chip, i.e., die 12B, is attached back-to-back to the flip-chip 12A (with an intervening insulation layer 24) and connected by wires 18 to the substrate 14. This particular MCM device 10 is illustrated in drawing FIG. 5.
In another form depicted in drawing FIG. 6, two chips 12A, 12B are mounted on opposite sides of a substrate 14, with intervening insulation layers 24. The dice 12A, 12B are shown with bond wires 18. This general dice-to-substrate configuration with variants is pictured in U.S. Pat. No. 5,147,815 to Casto, U.S. Pat. No. 5,689,135 to Ball, and U.S. Pat. No. 5,899,705 to Akram.
An MCM device 10 which combines various die configurations already described above in drawing FIGS. 1 through 6 is shown in U.S. Pat. No. 6,051,878 to Akram et al. The apparatus uses conductive column-like structures to connect substrates which carry the dice.
As shown in drawing FIG. 7, an MCM device 10 described in U.S. Pat. No. 5,483,024 to Russell et al. has two identical dice 12A, 12B with central bond pads. The dice are sandwiched between and attached to two lead frames 14A, 14B with discontinuous adhesive layers 16A and 16B. The dice 12A, 12B are joined by an intervening insulation layer 24. Bond wires 18 connect each die to the corresponding lead frame.
In drawing FIG. 8, a stacked MCM device 10 is depicted in accordance with the disclosure of U.S. Pat. No. 5,323,060 to Fogal et al. In this device, dice 12A, 12B, 12C, and 12D are vertically alternated with adhesive layers 16A, 16B and 16C. The thickness of the adhesive layers is enhanced to be greater than the bond wire loop height, so that bond wires 18 may be attached to the active surfaces of the dice, for connection to the substrate 14.
Described in U.S. Pat. No. 5,291,061 to Ball is a similar stacked device 10 in which the thickness of the adhesive layers 16A, 16B and 16C is reduced, using a low-loop-profile wire-bonding operation.
As shown in drawing FIG. 9, a device configuration generally shown in U.S. Pat. No. 5,399,898 to Rostoker uses an upper flip-chip die 12E to join dice 12A mounted on a substrate 14. The dice 12A are connected to substrate metallization with bond wires 18. Thus, the device 10 comprises three dice connected serially.
There are various forms of an MCM device in which separate enclosed units are first formed and then stacked. Examples are described in U.S. Pat. No. 5,434,745 to Shokrgozar et al. and U.S. Pat. No. 5,128,831 to Fox, III et al. A typical stacked device 10 of this construction is depicted in drawing FIG. 10, showing three units. Each unit comprises an intermediate substrate 15A with a metalized surface. A die 12A, 12B or 12C is mounted on the intermediate substrate 15A and connected to the metallization 30 by bond wires 18. A wall 32 surrounding each die 12 encloses the die 12, bond wires 18, and metallization 30. The various metallization leads extend to conductive columns 34 within the wall 32, the latter connected to metallization 40 on substrate 14. An insulative cover 38 protects the upper unit and forms a protective shell about the device.
In another design of MCM package device 10 shown in drawing FIG. 11, a plurality of dice 12A, 12B, . . . have beveled edges 28 which permit the bonding of wires to edge bond pads on the active surfaces 26. This design requires that the die thickness 36 be sufficiently great to accommodate wire loop height in the beveled regions. If the die thickness 36 is insufficient, the thickness of adhesive layers 16 must be increased. Thus, the device height will be increased. Also, the beveled edges 28 are weak and subject to breakage.
In each of the above prior art configurations for forming MCM devices containing a stack of identically configured dice, various limitations and/or problems exist as indicated above. A new device design is needed in which a plurality of identical dice with bond pads along one edge or two edges may be readily stacked for parallel operation. The new design must provide a device requiring fewer manufacturing steps and providing high density with enhanced reliability.